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| United States Patent |
5,057,433
|
|
Douglas
|
October 15, 1991
|
Indicator elements for autoclaves
Abstract
An indicator element which provides a visual indication when subjected to
correct steam sterilization conditions in an autoclave, the element
comprising a substrate having coated thereon an ink formulation comprising
as the primary visual change component: (i) a sulfur-containing compound
capable of decomposition to yield a metal sulfide under steam
sterilization conditions, or (ii) the precursors of which a
sulfur-containing compound which will yield said sulfur-containing
compound under aqueous conditions.
| Inventors:
|
Douglas; Malcolm F. (Swansea, GB3)
|
| Assignee:
|
Minnesota Mining and Manufacturing Company (St. Paul, MN)
|
| Appl. No.:
|
619657 |
| Filed:
|
November 27, 1990 |
Foreign Application Priority Data
| Current U.S. Class: |
436/1; 422/56; 422/57; 422/58; 422/119 |
| Intern'l Class: |
G01N 031/22 |
| Field of Search: |
463/1
422/56-58,119
|
References Cited
U.S. Patent Documents
| 2889799 | Jan., 1955 | Korpman | 116/114.
|
| 3360337 | Dec., 1967 | Edenbaum et al.
| |
| 3360338 | Dec., 1967 | Edenbaum.
| |
| 3360339 | Dec., 1967 | Edenbaum.
| |
| 3523011 | Aug., 1970 | Bhiwandker et al. | 436/1.
|
| 3981683 | Sep., 1976 | Larsson | 422/57.
|
| 4168779 | Sep., 1979 | Yokokoji et al. | 206/439.
|
| 4240926 | Dec., 1980 | McNeely | 436/1.
|
| 4857450 | Aug., 1989 | Burrows et al. | 430/603.
|
| Foreign Patent Documents |
| 1335076 | Jul., 1963 | FR.
| |
| 135156 | Apr., 1979 | DD | 436/1.
|
| 1132334 | Oct., 1968 | GB.
| |
| 1211603 | Nov., 1970 | GB.
| |
| 1458553 | Dec., 1976 | GB.
| |
| 2143322 A | Feb., 1985 | GB.
| |
Other References
Chemical Abstracts, vol. 93, 1980, p. 97, No. 206328s, Columbus, Ohio, U.S.
Lead Chemicals, p. 257, Lead Thiosulfate, International Lead Zinc Research
Organization, Inc. New York N.Y.
|
Primary Examiner: Johnston; Jill
Attorney, Agent or Firm: Kirn; Walter N., Boeder; Jennie G.
Parent Case Text
This is a continuation of application Ser. No. 160,149 filed Feb. 26, 1988,
now abandoned.
Claims
I claim:
1. A steam sterilization indicating ink formulation which when printed on a
substrate is capable of undergoing a visual change upon being subjected to
steam sterilization and which undergoes no visible change or a different
visual change upon being subjected to conditions milder than steam
sterilization, the formulation comprising a non-reactive binder and as the
primary visual change component:
(i) a sulfur-containing compound which is not a sulfide at ambient
conditions but is capable of decomposition to yield a metal sulfide under
steam sterilization conditions or,
(ii) a precursor of a sulfur-containing compound, which sulfur-containing
compound is not a sulfide at ambient conditions but is capable of
decomposition to yield a metal sulfide under steam sterilization
conditions, said precursor capable of yielding said sulfur-containing
compound under aqueous conditions.
2. A steam sterilization indicating ink formulation which when printed on a
substrate is capable of undergoing a visual change upon being subjected to
steam sterilization and which undergoes no visible change or a different
visual change upon being subject to conditions milder than steam
sterilization, the formulation comprising a non-reactive binder and as the
primary visual change component, a sulfur-containing compound which is not
a sulfide at ambient conditions, but is capable of decomposition to yield
a metal sulfide under steam sterilization conditions.
3. The ink formation of claim 1 wherein said visual change component
comprises said precursor of said sulfur-containing compound, and said
precursor comprises lead carbonate and sodium thiosulfate.
4. The ink formation of claim 1 wherein said visual change component
comprises a sulfur-containing compound and said full sulfur-containing
compound comprises a thiosulfate or polythiosulfate radical.
5. The ink formation of claim 1 wherein said visual change component
comprises a sulfur-containing compound and said sulfur-containing compound
is a thiosulfate selected from the group consisting of lead thiosulfate
selected from the group consisting of lead thiosulfate, copper
thiosulfate, silver thiosulfate, bismuth thiosulfate, cobalt thiosulfate,
chromium thiosulfate, nickel thiosulfate and ferrous thiosulfate.
6. The ink formation of claim 1 wherein said visual change component
comprises said precursor of said sulfur-containing compound and said
precursor comprises lead nitrate and sodium thiosulfate.
7. The ink formation of claim 1 wherein said binder 20 comprises
nitrocellulose.
8. The ink formulation of claim 1 wherein said visual change component
comprises a sulfur-containing compound and said sulfur-containing compound
comprises lead thiosulfate present in a concentration of between about 10
and 50 percent by weight of said ink formulation and said binder comprises
a nitrocellulose binder present in a concentration of between about 20 and
90 percent by weight of said ink formulation; and wherein said ink
formulation further comprises magnesium carbonate in a concentration of
between about 0 and 10 percent by weight of said ink formation, n-propanol
in a concentration of between and 0 and 30 percent by weight of said ink
formation and n-propyl acetate in a concentration of between about 0 and
30 percent by weight of said ink formation.
9. The ink formation of calim 8 wherein said lead thiosulfate is present in
a concentration of about 29 percent by weight of said ink formulation,
said magnesium carbonate is present in a concentration of about 1 percent
by weight of said ink formulation, said n-propanol is present in a
concentration of about 10 percent by weight of said ink formation, said
n-propyl acetate is present in a concentration of about 1 percent by
weight of said ink formulation, and said nitrocellulose binder is present
in a concentration of about 45 percent by weight of said in formulation.
10. A steam sterilization indicator comprising a substrate having coated on
at least a portion of one major exterior surface thereof a medium capable
of undergoing a visual change upon being subjected to steam sterilization
and which undergoes no visible change or a different visible change upon
being subjected to conditions milder than steam sterilization, wherein
said medium comprises as the primary visual change component:
(i) a sulfur-containing compound which is not a sulfide at ambient
conditions, but is capable of decomposition to yield a metal sulfide under
steam sterilization conditions or,
(ii) a precursor of a sulfur-containing compound, which sulfur-containing
compound is not a sulfide at ambient conditions but is capable of
decomposition to yield a metal sulfide under steam sterilization
conditions, said precursor capable of yielding said sulfur-containing
compound under aqueous conditions.
11. The indicator of claim 10 wherein said medium comprises as the primary
visual change component said precursor of said sulfur-containing compound,
which precursor comprises lead carbonate and sodium thiosulfate.
12. The indicator of claim 10 wherein said substrate is selected from
paper, saturated or coated paper, card, plastics material, metallized
material, metal foil, woven textile materials or non-woven textile
materials.
13. The indicator of claim 10 wherein said medium is overcoated with a
release coating and the opposite major surface of said substrate is coated
with a pressure sensitive adhesive.
Description
FIELD OF INVENTION
The present invention relates to colour change indicator elements and in
particular to colour change indicator elements which effect a colour
change under sterilization conditions of high temperature and humidity
effected during steam sterilization.
BACKGROUND OF THE INVENTION
In hospitals, clinics and the like, it is standard practice to sterilize
various products such as gowns, drapes, sheets, dressings, and other
articles, prior to use by placing them in an autoclave where they are
subjected to steam sterilization. This practice is necessary to avoid
infection and prevent contamination from the use of such articles where
the same are not in a sterile condition and it is particularly important
where the articles have previously been used in the care of other
patients. As there is no visual way of determining whether a particular
article is sterile or not it has been the practice to use, with the
article, when placed in the autoclave, a colour change indicator element
which changes colour under the sterilizing conditions of the autoclave,
thus indicating that the particular article or package has been passed
through the sterilizing cycle. The indicator may be in the form of a
ribbon or card to which a colour change ink has been applied.
It is generally the practice in sterilizing such articles to gather several
articles together, bundling the same in a porous wrap, and then the
package, held together by tying with string or by pressure-sensitive
adhesive tape, is placed in an autoclave together with a sterilizing
indicator element, the sterilization indicator element may be either
inserted in or applied to the package. Where pressure-sensitive adhesive
tapes are used for this purpose, it is convenient to have the colour
change indicator on the tape backing. By observation of the visible back
of the tape which holds the package together, one can readily determine by
its colour change whether or not the same has been passed through the
sterilization cycle. Pressure-sensitive adhesive tapes of this type are,
for example, described in U.S Pat. No. 2,889,799.
Typical sterilization conditions comprise subjecting the articles to
saturated steam at a temperature of between 132.degree. C. and 140.degree.
C., usually between 134.degree. C. and 140.degree. C. for a period of not
less than 3 minutes. Another steam sterilization cycle comprises
subjecting articles to 121.degree. C. for not less than 15 minutes. In
order to ensure correct sterilizing conditions, the steam must penetrate
unhindered to the centre of the load. This can be achieved only if all of
the air is first removed from the sterilizer vessel and its load and this
is accomplished typically by a process of evacuation and steam flushing of
the sterilizing vessel and its load.
Failure to remove all of the air or the subsequent leakage of air into an
evacuated chamber, or the introduction of air or non-condensible gases in
the steam supply, causes gas pockets to remain within the porous load,
usually in the inner regions thereof. In this case the temperature within
the load might be lower in some places than that required during all or
part of the sterilization process.
A standard test for the efficiency of air removal and/or the inclusion of
non-condensible gases in autoclaves is known as the Bowie/Dick towel test.
This test utilises a stack of standard linen Huckaback towels measuring
some 270 mm high and about 300 mm .times.200 mm in plan. At about the
vertical centre of the stack there is placed a sheet of paper bearing
so-called autoclave tape or other indicator which undergoes a change in
appearance in the presence of certain levels of moisture and temperature.
The test pack is processed in the autoclave and the satisfactory result
would show an even change in appearance across the whole of the indicator
sheet, whereas the presence of air or non-condensible gas in the stack is
indicated by a failure of the indicator to change its appearance in
certain areas, usually at the centre. This test must be carried out daily
before the autoclave is used for the production of sterile porous loads.
There are certain disadvantages associated with this known test. For
example, the standard towel stack is too large to fit into many
small-chambered autoclaves. Furthermore, the Huckaback towels are
expensive and usually require to be laundered at least once a week. The
towels require to be aired carefully between tests and they deteriorate
gradually and become unusable within about 12 months of normal use. This
is because the fibrous consistency becomes matted so that steam cannot
readily diffuse through it, thus increasing the risk of misleading test
results. Furthermore, Huckaback towels, being made from natural fibre, can
give rise to exothermic reactions, i.e. they will absorb moisture very
readily with the release of heat, when they are too dry. In the case of
the Bowie/Dick test such a phenomenon can result in the temperature within
the pack being higher than the temperature in the autoclave chamber and
again this can lead to misleading results.
British Patent Specification No. 2143322A discloses a test kit for
detecting the presence of air in a steam sterilizer which comprises a
first porous mass of at least substantially man-made material, a second
porous mass of a similar material, an indicator adapted to undergo a
visual change under moist heat sterilising conditions and sandwiched
between the masses thus to be in intimate contact therewith, and means for
removably holding the masses and indicator in close superimposed
relationship, said means being permeable to allow the free passage of air
and steam to the external surfaces of said masses.
The test kit is generally in the form of a cuboid having a side dimension
in the region of 12 to 15 cm and the indicator may conveniently comprise
an autoclave tape or printed sheet having indicator in the shape of a St.
Andrews cross or other pattern extending from side to side.
Experimental work has demonstrated that such a device as described is
capable of detecting the presence of air in an autoclave. It is also clear
from temperature measurements made within the device that the presence of
air leads to a lower temperature at the centre of the test pack similar to
that observed with the Huckaback towel pack used in the conventional
Bowie/Dick test. The presence of air in the autoclave results in uneven
change in the indicator, often incomplete colour change towards the centre
of the device.
One of the most commonly used colour change media for use as an indicator
contains sulfur and lead oxide. The sulfur in the presence of the lead
oxide, under the conditions of the steam sterilization, changes from a
yellow to a black colour as it is converted to lead sulfide. It has been
the practice to mix the sulfur and the lead oxide together in equivalent
molar proportions in an ink base which is then applied to an indicator
card or to a back of a pressure-sensitive adhesive tape, which, in turn,
is then associated with the articles to be sterilized. If the autoclave or
other steam sterilizing equipment used is not functioning, or if the
package to be sterilized inadvertently gets omitted from being placed in
the autoclave, this can readily be determined by the fact that no reaction
has occurred between the sulfur and the lead oxide as shown by the lack of
colour change in the indicator. Similarly if air or non-condensible qas is
present in the autoclave an incomplete reaction will occur in some areas
resulting in incomplete generation of the black colour.
One difficulty with such inks is the tendency to stain when in contact with
many articles. Staining can be the result of the transfer and reaction of
sulfur as the formation of soluble materials during the colour change
reaction staining causes unsightly marks on sheets and other articles
which may prevent their use. Staining can be reduced by the provision of
an overcoat over the printed ink requiring an extra processing step in the
production of indicator elements thereby increasing the cost.
Another difficulty with the conventional sulfur lead oxide colour change
inks is that they are relatively unstable, necessitating the practice of
forming separate solutions of the sulfur and the lead oxide and then
combining the two together just prior to using the ink for marking. This
is apparently due to the fact that the sulfur and lead oxide tend to react
while in the solvent medium used in forming the ink. If the ink solution
is prepared several days prior to its use, a substantial reduction in
colour change sensitivity results.
Where the colour change indicator is to be used in combination with a
pressure-sensitive adhesive tape, as by printing the ink on the back of
the tape, the inks containing the sulfur have the further disadvantage
that the sulfur in the marking tends to migrate into the
pressure-sensitive adhesive where the tape is wound on itself in roll
form, thus further reducing the sensitivity of the marking to colour
change, when the tape, which the ink marking thereon, is later exposed to
steam sterilization conditions.
Although giving a good colour change from a light yellow to a deep black
with fresh inks, the colour change indicator markings made from the active
ingredients, lead oxide and sulfur, tend to lose much of their sensitivity
over extended periods of storage. This is believed to be due to the
oxidation of sulfur and is particularly prevalent under relatively hot
conditions. Markings may not turn to the same deep black on being
subjected to sterilizing conditions but tend to assume a dark grey
appearance, with the result that the colour change is not nearly as marked
as with fresh inks. This is particularly disadvantageous when the
indicators are being used in testing the performance of an autoclave
utilising the Bowie/Dick or similar test as misleading results may be
obtained.
Despite these inherent disadvantages in colour change media using sulfur
and lead oxide, such markings have continued to be widely used as steam
sterilization indicators.
British Patent Specification No. 1211603 discloses an indicator material
for determining whether steam sterilization has been applied comprising a
mixture of lead carbonate and calcium sulfide. In the presence of steam at
a temperature of about 120.degree. C. the calcium sulfide decomposes to
form calcium hydroxide and hydrogen sulfide. The hydrogen sulfide in turn
attacks lead carbonate to form black lead sulfide and carbonic acid.
Other known colour change indicators for indicating steam sterilization
include e.g. the use of copper sulfite disclosed in U.S. Pat. No. 3 360
339, which provides a colour change from red to green under steam
sterilization conditions, and have not found commercial favour.
It has now been found that there are a number of sulfur-containing
compounds which will decompose to yield a sulfide under steam
sterilization conditions with a marked colour change.
SUMMARY OF THE INVENTION
According to the present invention there is provided an indicator
comprising a substrate having coated thereon a formulation capable of
undergoing a visual change upon being subjected to steam sterilization and
which undergoes no visible change or a different visible change upon being
subjected to conditions milder that steam sterilization, the formulation
comprising as the primary visual change component: (i) a sulfur-containing
compound capable of decomposition to yield a metal sulfide under steam
sterilization conditions or, (ii) the precursors of such a
sulfur-containing compound which will yield said sulfur-containing
compound under aqueous conditions.
Also according to the invention there is provided an ink formulation which
when printed on a substrate is capable of undergoing a visual change upon
being subjected to steam sterilization and which undergoes no visible
change or a different visual change upon being subjected to conditions
milder than steam sterilization, the formulations comprising a binder and
as the primary visual change component (i) a sulfur-containing compound
capable of decomposition to yield a metal sulfide under steam
sterilization conditions, or, (ii) the precursors of such a sulfur
containing compound which will yield said sulfur containing compound under
aqueous conditions.
It has been found that there are a number of compounds having
sulfur-containing radicals which will decompose to the metal sulfide under
steam sterilization conditions with a pronounced colour change. Metal
sulfides tend to be strongly coloured and are often the most stable form
of metal sulfur-containing compounds. Furthermore, they are often
insoluble in water and may be held in a binder to prevent staining. The
preferred sulfur-containing radical is thiosulfate although other groups
may be employed e.g. polythionates etc. Compounds for use as the primary
colour change component include:
lead thiosulfate which is white in colour and decomposes to yield black
lead sulfide under steam sterilization conditions,
copper thiosulfate which is yellow in colour and decomposes to yield black
copper sulfide under steam sterilization conditions,
ferrous thiosulfate which is light green in colour and decomposes to yield
a black sulfide under steam sterilization conditions,
nickel thiosulfate which is light green in colour and decomposes to
black/green nickel sulfide under steam sterilization conditions,
cobalt thiosulfate which is light red/purple in colour and decomposes to
deep purple/black cobalt sulfide under steam sterilization conditions,
bismuth thiosulfate which is orange/brown in colour and decomposes to black
bismuth sulfide under steam sterilization conditions,
chromium thiosulfate which is grey/blue in colour and decomposes to dark
green chromium sulfide under steam sterilization conditions,
silver thiosulfate which is brown in colour and decomposes to black silver
sulfide under steam sterilization conditions.
It is also possible to employ precursors of such sulfur-containing
compounds which will yield the sulfur-containing compounds under aqueous
conditions. For example lead carbonate and sodium thiosulfate may be
employed as the colour change component in the ink, these compounds
undergoing a double decomposition reaction to yield lead thiosulfate under
aqueous conditions. During the steam sterilization cycle lead thiosulfate
is initially formed which then decomposes to lead sulfide providing the
desired colour change.
The sulfur containing compounds or their precursors used in the invention
may be employed as the sole colour change component and may be combined
with a binder and solvent in an ink formulation. Such ink formulations may
be readily prepared and tend to be physically and chemically stable on
storage since the sulfur-containing compounds are stable at ambient
temperature. The sulfur-containing compounds may also be used in
combination with other components which aid or modify the colour
generation under steam sterilization conditions. Other additives include
carbonates, particularly lithium carbonate and magnesium carbonate. Such
compounds may be used in formulation in amounts up to that of the lead
thiosulfate. The selection of these materials is dependent upon the
particular property required by the indicator tape. For example, it may be
desirable to provide an indicator tape which will not undergo a
substantial visual change unless it has been subjected to steam
sterilization conditions for a certain period of time, e.g. two minutes.
The binder system for the ink formulations may comprise a film-forming
carrier which is permeable to steam in order to obtain a satisfactory
colour change under steam sterilization conditions. Binder systems based
on nitrocellulose have been found to be particularly effective but many
other binder systems may be used e.g. binder systems cured by ultraviolet
irradiation or electron beam bombardment e.g. urethane-acrylates,
epoxyacrylates and polyester acrylates. Other binder systems include vinyl
resins e.g. copolymers of polyvinyl chloride and polyvinyl acetate as
disclosed in U.S. Pat. No. 3360339.
The printing ink formulations ma contain other additives, e.g. solvents,
defoamers, flow aids and other printing aids.
The formulations are prepared by admixing the components of the binder
system, solvent and colour change chemistry, e.g. in a mixer, ball mill or
attriter. The binder system generally comprises from 50 to 98% by weight
of the ink formulation and the colour change chemistry from 2 to 50% by
weight of the ink formulation, usually in the range 20 to 35% by weight of
the ink formulation.
A preferred ink formulation of the invention comprises
______________________________________
general range
preferred
% by weight
% by weight
______________________________________
lead thiosulfate
10 to 50 29
magnesium carbonate
0 to 10 1
n-propanol 0 to 30 10
n-propyl acetate
0 to 30 14
nitrocellulose binder
20 to 90 45
______________________________________
The indicators of the invention may oomprise any substrate which is capable
of withstanding steam sterilization conditions, i.e. a temperature of 121
to 138.degree. C. in the presence of steam for a period of up to 30
minutes. Suitable substrates include paper which may be absorbent or
saturated with a rubber/resin solution or a natural or synthetic latex,
coated paper, card, plastics material, metallised material, metal foil and
non-woven or woven textile materials.
In one preferred form, the substrate is in the form of a tape, more
preferably a pressure sensitive adhesive tape having a release coating
upon one surface and a pressure sensitive adhesive on the other such that
the tape may be wound in the form of a roll.
A second preferred form comprises a rectangular or square test sheet having
an area on which the indicator has been applied of a side dimension in the
range 10 to 15 cm for use in a Bowie/Dick test or Lantor cube of the type
disclosed in British Patent Specification No. 2143322A.
A particularly preferred form for use with the Lantor cube (commercially
available from 3M UK PLC) comprises a paper sheet 30 cm by 15 cm, one half
of the sheet having the indicator ink printed thereon (eg. in a pattern
comprising a cross formed by diagonal lines extending from two corners and
a series of chevrons formed of lines parallel to the cross) and the second
half bearing printed matter for recording details of the test, e.g. date,
hospital, machine number, test results etc. The indicator paper is folded
in half and placed between th blocks of the Lantor cube during testing.
Other substrates may be in the form of a bag or other wrapping in which
case the printing ink formulation may be confined to small areas.
The ink formulations may be printed by a range of printing techniques, e.g.
flexografic, rotogravure and screen printing, etc. The ink is generally
applied in the form of patterns, e.g. stripes, chevrons, etc., in order to
provide a visual contrast between areas of the indicator which will
provide a visual change after steam sterilization and background areas of
the indicator. However, the use of patterns is not essential and the
indicator may be completely coated, e.g. by a web coating technique.
The invention will now be illustrated by the following Examples.
EXAMPLE 1
PREPARATION OF LEAD THIOSULFATE
Concentrated nitric acid was added to distilled water to form a dilute
solution. Lead carbonate was added slowly and the CO.sub.2 effervescence
allowed to die down before more was added. Lead carbonate was continually
added until no more effervescence was observed. This indicated that a near
neutral solution of lead nitrate was obtained. A solution of sodium
thiosulfate was made up and when all of the crystals had dissolved, this
solution was poured into the solution of lead nitrate. Immediately a white
precipitate of lead thiosulfate was formed. It was allowed to settle, the
liquor poured off and fresh distilled water was stirred in, the lead
thiosulfate allowed to settle and the liquor again poured off. This
washing process was again repeated. The slurry was then filtered through a
Buchner funnel, the precipitate washed with more distilled water and then
ethanol. Finally a wash with diethyl ether was made and the air pulled
through the lead thiosulfate by the vacuum to dry off the ether.
EXAMPLE 2
Three stock ink formulations were made up to the following formulation,
which ingredients are expressed as parts by weight:
______________________________________
Ingredient/Ink A B C
______________________________________
lead thiosulfate 180 270 360
n-propanol (solvent)
103 103 103
n-propylacetate (solvent)
155.5 155.5 155.5
nitrocellulose binder
476 476 476
Modaflow 3.1 3.1 3.1
Byke 141 1.6 1.6 1.6
______________________________________
The nitrocellulose binder used is commercially available from Fishburn
Inks, Watford, England and is approximately 30% solids.
Modaflow is a trade mark for a flow aid commercially available from
Monsanto.
Byke 141 is a trade mark for a defoamer commercially available from
Byke-Chemie GmbH, Wesel, Germany.
The two solvents were added to the container of a laboratory air driven
mixer, the mixer started and the lead thiosulfate added slowly. When this
had evenly dispersed the binder was slowly poured in followed by the
Modaflow and the Byke 141. The ink was then mixed for 20 minutes before
use.
Each of the three inks was printed onto 80 gsm photocopy paper using a
gravure foil, and dried at a temperature of 50.degree. C.
Inks A1, B1 and C1 were prepared from inks A, B and C above by the addition
of magnesium carbonate to each of the formulations in an amount of 12
parts by weight. Each of the inks A1, B1 and C1 were printed as described
above.
Inks A2, B2 and C2 were prepared from inks A, B and C above by the addition
of magnesium carbonate to each of the formulations in an amount of 24
parts by weight. Each of the inks A2, B2 and C2 were printed as described
above.
Printed samples obtained with each of the ink formulations above were
placed in a Lantor cube which was placed in an autoclave and subjected to
steam sterilization for 3 minutes at 134.degree. C.
The Lantor cube comprised two fibrous blocks essentially made of spun
bonded polypropylene-sheets layered one upon another and held together in
intimate contact by means of a porous shrink wrap film, said fibrous
blocks being 15 cm by 15 cm by 7.5 cm. The two fibrous blocks are held
together in intimate contact with the desired indicator between by means
of a stainless steel clamp assembly which comprises two square end plates
and two side panels which act as compression springs. The Lantor cube and
clamp assembly are commercially available from 3M UK PLC.
In each case the printed samples exhibited a clear colour change from white
to black and no staining of the cube took place. The clearest and darkest
samples were those printed with formulations A2, B1 and B2.
Printed samples using inks B1 and B2 were placed in a Lantor cube and
subjected to autoclaving as described above with the exception that air
was deliberately introduced into the autoclave in an amount sufficient to
provide a temperature difference of 0.5.degree. C. between the drain and
the centre of the Lantor cube. This experiment was designed to represent a
faulty autoclave having an air leak. The resulting samples exhibited full
colour change at the edges of the sheet with areas of incomplete colour
change towards the centre thus indicating incomplete sterilization
conditions.
EXAMPLE 3
The following ink formulation was prepared
______________________________________
copper thiosulfate 300 g
magnesium carbonate 12 g
n-propanol 103 g
n-propyl acetate 156 g
nitrocellulose binder 476 g
Modaflow 3.1 g
Byke 141 1.6 g
Total 1051.7 g
______________________________________
The ink was made exactly as described previously and printed as before onto
photocopy paper. Testing entailed placing a 150 mm square sample into the
Lantor cube and subjecting to steam sterilization at 134.degree. C. for 3
minutes.
It was noted that copper thiosulfate is apparently slightly unstable. When
initially made it was a canary yellow colour, but when printed it was a
yellow-light olive green colour which turns into a dense-black colour on
autoclaving in the Lantor cube.
EXAMPLE 4
LEAD NITRATE AND SODIUM THIOSULFATE AS THE COLOUR CHANGE COMPONENTS IN AN
AUTOCLAVE INK
An autoclave ink of the following formulation was made by milling all of
the components in a ball mill for about 16 hours.
______________________________________
Lead nitrate 312 g
Sodium thiosulfate 233 g
Magnesium Carbonate 12 g
Nitrocellulose binder 476 g
n-propanol 103 g
n-propyl acetate 156 g
Modaflow 3.1 g
Byke 141 1.6 g
______________________________________
The ink was printed onto 80 gsm photocopy paper as previously described and
then subjected to development in an autoclave for three minutes at
134.degree. C. with a sample of the sheet in a Lantor cube and in a stack
of Huckaback towels as used for a Bowie and Dick test.
A clear colour change from white to black was observed in each sample.
EXAMPLE 5
EVALUATION OF ELEMENTAL THIOSULFATES PRIMARY COMPONENT FOR VISUAL CHANGE
INDICATING MEDIUM
The following materials were each weighed out as indicated and dissolved
separately in distilled water in a test tube. A number of other test tubes
containing 5g of sodium thiosulfate in distilled water were prepared. When
the materials were totally in solution the thiosulfate was poured in turn
until each of the other solutions to form the respective elemental
thiosulfate, either in solution or as a precipitate. Bismuth thiosulfate
was precipitated washed and dried to an orange/light brown powder.
Each solution was then boiled for a few minutes and the colour changed
observed. Results are indicated in the following Table. In the case of
bismuth thiosulfate, the solid was added to a test tube with some
distilled water and was then boiled as before.
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COLOUR/PRECIPITATE
QUANTITY
WITH SODIUM COLOUR/PRECIPITATE
MATERIAL g THIOSULFATE WITH BOILING
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Cerous nitrate
5.83 milky white yellow precipitate
Caesium chloride
8.78 water white water white
Cobaltous nitrate
5.86 light red/purple
deep purple/black
Bismuth nitrate
9.77 orange/brown precipitate
black precipitate
Stannous chloride
4.55 yellow/white yellow/white
Barium nitrate
5.26 white precipitate
off white precipitate
Manganous chloride
3.98 water white milky solution
Chronium nitrate
5.37 grey/blue (milky)
dark green
Nickel nitrate
5.86 light green black/green
Ferrous sulfate
5.6 light green black
Silver nitrate
6.84 brown precipitate
black precipitate
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It can be seen from the Table that cobalt, bismuth, chromium, nickel,
ferrous and silver thiosulfates provide a pronounced visual change and
could be useful in an indicator ink system. The precursors of these
compounds may also be used as a colour change component in indicator inks.
EXAMPLE 6
LEAD TETRATHIONATE AS COLOUR CHANGE COMPONENT N AN AUTOCLAVE INK
Sodium thiosulfate solution was slowly added to a solution of blue
capricnitrate trihydrate until the blue colour just disappeared. A
solution of lead nitrate was then added to give an off-white precipitate
of lead tetrathionate. The precipitate was allowed to settle, the liquor
poured off and distilled water added with stirring. The precipitate was
filtered, washed again with distilled water, methylated spirits and
finally with ether and thereafter dried.
50g of the lead tetrathionate was dispersed in 100g of the nitrocellulose
binder used in Example 2 and the composition was hand spread onto sheets
of 80gsm photocopy paper using No. 1 and No. 8 K Bars. The sheets were
dried for 5 minutes at 50.degree. C. and then autoclaved for 3 minutes at
134.degree. C. All samples developed a dark black colour.
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